first of all I wish to say thanks for your feedback on my posts :-)
So now the groupware is up and running and the next steps are planned already (XMPP = Chat/Messaging, OwnCloud).
The roundcube web frontend is a bit slow for daily work, and it reconnects to the imap server very often.
Sending mails from my own domain (dynamic IP) uses a smart host (mailjet, costs less and 200 mails/day are free :-)) and receiving mails also takes sometime because of Spam/Virus detection.
So the next parts are also performance improving.
Some topics to check are
memory usage (count of server process fro IMAP, Apache ...)
Proxy for roundcube connection to IMAP
deactivating Virus/Spam detection and use some header/body checks instead (not for production use of course)
If you have more cheats and hints to increase the performance, that would be nice.